1. Field of the Invention
The present invention relates to a temperature distribution measuring apparatus which projects pulsed-light into an optical fiber, measures the Raman spectrum of backscattered light occurring in the optical fiber, and determines a temperature distribution along the optical fiber.
2. Description of the Related Art
In the field of optical sensing technology, a temperature distribution measuring apparatus using optical time domain reflectomery (OTDR) techniques is known which allows pulsed-light to enter an optical fiber at one end, samples and measures the Raman spectrum of backscattered light occurred at various points in the optical fiber, and determines from the sampling data a temperature distribution along the optical fiber.
FIG. 1 is a schematic diagram illustrating the measuring principle of a conventional temperature distribution measuring apparatus of this type. A laser pulse enters an optical fiber 100 to excite backscattered light in the optical fiber 100. The spectrum, intensity, and polarization state of the backscattered light contain temperature information at the point where the backscattered light has occurred. The backscattered light that propagates toward the laser-pulse incident side is sensed and processed as a time-sequence signal. A one-dimensional temperature distribution along the optical fiber is thus measured.
The backscattered light occurring when a laser pulse enters the optical fiber 100 includes the Rayleigh spectrum due to fluctuations in density, the Brillouin spectrum due to propagative fluctuations, and the Raman spectrum due to rotation and vibration of molecules. The Brillouin spectrum and the Raman spectrum are inelastic scattered light and have a different spectrum from that of the excited light. The temperature information is contained in all three types of scattered light. The most temperature-sensitive spectrum is the Raman spectrum, whose intensity varies with temperature.
When the temperature is measured using the Raman spectrum, the Stokes component of the Raman spectrum of the backscattered light whose wavelength becomes longer than that of the incident light and the anti-Stokes component of the Raman spectrum of the backscattered light whose wavelength becomes shorter than that of the incident light are extracted by an optical filter, and the temperature distribution is calculated based on the ratio between the two components of backscattered light. Of course, the measurement can be performed on the basis of only one component of the Raman spectrum of the backscattered light. In both cases, however, at least one component of the Raman spectrum of the backscattered light must be extracted by an optical filter. In such a temperature distribution measuring apparatus, not only the accuracy of temperature, a physical quantity to be measured but also its positional resolution is important. The positional resolution is generally determined by the incident pulse width. It further depends on the sampling frequency when the processing system of the backscattered light is a sampled-value system.
If the sampling period is sufficiently shorter than half the width of the incident light pulse, the measured backscattered light the total of the backscattered light for half the width of the incident light pulse. Therefore, the measured temperature is the average temperature over half the pulse width. Even if the sampling frequency becomes fast, the positional resolution cannot be increased more than the width of the incident pulse.
Therefore, to increase the positional resolution, narrowing the pulse width of the incident light is effective. To achieve this, two methods can be considered: one is to actually narrow the width of the incident pulsed-light itself, and the other is to consider a response of the backscattered light to a pulsed-light of a finite width to be a transformation, and to obtain an impulse response by performing an inverse transformation of the measured value of the backscattered light.
Since narrowing the pulse width alone cannot make the width of a light pulse narrower than the pulse width determined by the rising and falling characteristics of a source of pulsed-light, it is difficult to make the width of the incident pulse narrower than that achieved by the elements already developed. Thus, this method is unsuitable for increasing the positional resolution.
In contrast, a method of obtaining an impulse response is considered effective in increasing the positional resolution.
Therefore, the method of obtaining an impulse response will be examined.
Since the transformation used in the method of obtaining an impulse response is a convolution in a range of linear approximation, if the impulse response is expressed as h(t) and the incident pulsed-light is expressed as P(t), the backscattered light signal g(t) will be obtained by performing convolution of the impulse response h(t) with respect to the incident light pulse P(t). It can be expressed as: ##EQU1##
Then, the impulse response h(t) can be obtained by measuring the incident light pulse P(t), an input signal, and performing deconvolution of the measured backscattered light signal g(t) with respect to the incident light pulse P(t).
Actually, however, even if deconvolution is performed on the measured data, it is difficult to obtain the correct impulse response because the measured signal contains noise components. The reason for this will be described below.
By achieving the Fourier transform of Equation (1) and adding noise component N(.omega.) to the backscattered light, the following equation will be obtained: EQU G(.omega.+N(.omega.)=H(.omega.)P(.omega.) (2)
For inverse transformation, by dividing both sides of Equation (2) by P(.omega.), the following equation is obtained: EQU H(.omega.)=G(.omega.)/P(.omega.)+N(.omega.)/P(.omega.) (3)
Here, the second term on the right side of Equation (3) is a problem. While the spectrum of incident light pulse component P(.omega.) is finite, the spectrum of noise component N(.omega.) extends over a very wide range, Thus, the division result diverges in the high-frequency range (where the angular velocity .omega. is large).
This divergence makes it impossible to obtain the impulse response correctly, so that the positional resolution cannot be improved as expected, thus reducing the total accuracy of position measurement.